Spacecraft, such as scientific and telecommunication satellites, contain high power equipment that generates significant amounts of heat which needs to be dissipated away from the equipment. Such equipment is conventionally mounted on structural panels of the satellite which may comprise carbon fibre reinforced plastic (“CFRP”) sandwich panels. These panels comprise a honeycomb core sandwiched by a CFRP skin on either side thereof. Such panels are very light and strong, providing excellent structural strength, but are poor conductors of heat and so controlling the temperature of such high power equipment can be problematic. Accordingly, it is necessary to provide separate means for heat dissipation. This may comprise “thermal doublers” which are thicker sheets of CFRP than those of the skin of the panel, to improve the thermal conductivity. Alternatively, devices known as “heat pipes” may be used.
Heat pipes comprise metallic (usually aluminium) pipes containing a thermally conductive fluid and which are thermally coupled to the heat-generating equipment and are mounted to the surface of the CFRP panels and extend away from the equipment such that heat can be conducted away from the equipment and dissipated to ambient surroundings. Heat pipes are more thermally efficient than thermal doublers which require more total mass than heat pipes to control the same temperature for a given power dissipation. Heat pipes may comprise a loop heat pipe system in which coolant is pumped around a closed loop of the heat pipe system, requiring a pump and active control electronics which add complexity, expense and additional mass to the equipment. Alternatively, the heat pipes may comprise passive heat pipes which are sealed pipes containing a coolant fluid which passively conducts heat along the heat pipe away from the heat source to which it is thermally coupled. Passive heat pipes are cheaper, simpler and lighter than heat pipe loop systems.
The metallic heat pipes, by necessity, are good conductors of heat, whereas the CFRP panels to which they are mounted are poor conductors of heat. Additionally, the coefficient of thermal expansion (“CTE”) of metallic heat pipes differs significantly to that of CFRP. Metallic heat pipes expand significantly more upon heating than do CFRP panels. CFRP panels show very little expansion upon heating, making them good for mounting of equipment that requires accurate positioning and alignment. Previous attempts to mount heat pipes to CFRP panels have involved bonding the heat pipes to the CFRP panel surface. However, when in use, the adhesive bonds are prone to failure and cracking under “excursion temperatures”, that is, when subjected to temperatures significantly above or below ambient/assembly temperature, due to the high thermo-elastic stress generated by the metallic heat pipes contracting and expanding to a greater degree than the CFRP panels to which they are attached. This differential expansion results in forces within the structure urging the CFRP panel from a planar shape into a curved shape, fracturing the adhesive bond, and the heat pipe may come away from and/or fracture the CFRP panel skin. Aside from weakening the attachment of the heat pipes to the CFRP panels, a cracked adhesive bond also has a greatly reduced thermal conductivity than a bond that is intact, which greatly reduces the heat transfer capacity of the adhesive between the CFRP panel and the heat pipe. The thermo-elastic loads and peak stresses generated in the heat pipe are generally concentrated at the remote ends of the heat pipe and create a peeling force acting to separate the ends of the heat pipe from the CFRP panel surface, and a shear force between the heat pipe and the CFRP panel surface.